In a typical internal combustion engine, the valve movements are controlled mechanically by a camshaft, with predetermined and unchangeable movements. This type of system is limited in that the valve lift and timing is not optimal for every engine operating condition, causing increased parasitic losses which are primarily due to air throttling. Also, this limitation may cause a non-stoichiometric air/fuel ratio. The stoichiometric value is typically an air/fuel ratio of 14.6 pounds air per 1 pound of fuel. This proper air/fuel ratio is critical in minimizing harmful pollutants in the exhaust, such as NO.sub.x, CO and HC, especially when the air pollution reduction is accomplished via a three way catalyst.
As a result, advances have been made that allow for variable valve lift of the individual valves, in order to better optimize the valve timing and lift schedules over the complete engine operating range, resulting in reduced throttling losses and improved air/fuel ratios. Examples of this are the lost motion type of system as shown in U.S. Pat. No. 4,930,465 Wakeman et al., or the electronic actuation of valves as shown in U.S. Pat. No. 4,009,695 Ule, or other variable valve lift systems known in the art.
While this type of system improves the fuel economy and performance for various operating conditions, differences may exist in the valve lifts between cylinders. Hence, the variation of time dependent valve open area between cylinders would result in differences in air flow between cylinders due to the intake process or differences in residual gas content between cylinders due to the exhaust process. A sensor or sensors could be added to monitor the valve lift cylinder to cylinder. However, the problem with this concept is two fold. First, even though key valve lift parameters may indicate equal lift, i.e., valve open time, close time and maximum lift, the shape of the lift curves could be different from cylinder to cylinder yielding different time dependent area during the intake process and hence different air flows. Second, addition of these sensors would involve extra cost and would require equalized calibration cylinder to cylinder to be accurate.
Without a valve lift equalization means, only an average valve lift correction can be applied to all cylinders equally, based on an average air/fuel ratio among the cylinders. In addition to differences in a flow caused by a variable valve cam system, there are additional variations of air/fuel ratios between the cylinders, due to manufacturing tolerances, induction system dynamics, fuel injector clogging and temperature effects. Therefore, the air/fuel ratio may not be optimum for a given cylinder even though the average among the cylinders is the desired value. Further, the average valve lift correction could produce significant air flow differences among the cylinders due to different shapes of the lift curves. This would result in torque differences among the cylinders which will increase noise, vibration and harshness.
One approach to maintaining the desired air/fuel ratios is described in U.S. Pat. No. 4,962,741 Cook et al. In this approach, a time resolved exhaust gas oxygen sensor placed in the combined exhaust stream, i.e., the exhaust stream containing exhaust from all cylinders, generates a signal which is read by a controller. This controller identifies the individual cylinder contributions to the total air/fuel ratio and correlates these with a corresponding combustion event. The controller will then generate a fuel pulse width signal for each individual electronically actuated fuel injector coupled to its corresponding cylinder, thus allowing each cylinder to operate at a desired air/fuel ratio. Once a variable valve system is incorporated, the potential for significantly larger variations in air flow among the cylinders increases to such an extent that mere fuel injector pulse width adjustments, even if they have the ability to adjust the air/fuel ratio to the correct level, may cause significant torque differential among the cylinders due to these air flow variations among the cylinders. The variable valve system removed the compromises present with the cam driven valve train; however, the ability to control valve motion introduces the need for some means of feed back control to be assured that the valve control system was delivering an equal amount of air to each cylinder and leaving an equal amount of residual gasses.
The invention herein recognizes the limitations of the prior art and combines variable valve lift capabilities into a system along with the capabilities of a time resolved oxygen sensor and controller to provide the capability to adjust the air/fuel ratios between the individual cylinders, to maintain equal indicated mean effective pressure levels in each cylinder, to a greater extent should engine operating conditions and air/fuel maldistribution due to manufacturing tolerances warrant. This can be done without the need for any sensors to directly measure the valve lift in any of the variable lift valves, which will reduce cost to the system and may be more reliable because a single sensor or dual sensors are used versus multiple sensors mounted on each valve. In addition, this system can be further combined with the electronically actuated fuel injectors to provide for even more flexibility in maintaining the proper equal air flow among the cylinders and proper air/fuel ratio within each individual cylinder when the valve lift is at a maximum, then the fuel injector pulse width variations can be used to obtain the desired air/fuel ratios within each cylinder.